Beach Bingo - Provably Fair

Last updated: March 1, 2026

1. How Randomness Works

Switchboard VRF (Verifiable Random Function)

Beach Bingo uses Switchboard On-Demand for all randomness. Switchboard is a decentralized oracle network that generates cryptographically verifiable random numbers. Here is how it works:

1. Request: When a game round starts, the program requests randomness from a Switchboard oracle
2. Generation: The oracle generates a random value using a VRF (Verifiable Random Function), which produces both a random output and a mathematical proof
3. Verification: The Solana program verifies the proof on-chain before accepting the random value. If the proof is invalid, the randomness is rejected
4. Derivation: Bingo numbers are derived from the verified random value using SHA-256 hashing with rejection sampling to ensure uniform distribution

Number Derivation

Each bingo number is derived as follows:

hash = SHA-256(vrf_result || round_id || number_index || attempt)
number = hash[0..2] as u16  // First 2 bytes as 16-bit integer
if number > max_valid:       // Rejection sampling (no modulo bias)
    increment attempt, retry
final_number = (number % 75) + 1  // Range: 1-75

Rejection sampling eliminates modulo bias, ensuring every number from 1-75 has an exactly equal probability of being selected.

2. Anti-Manipulation Protections

VRF Pre-Binding

The VRF account is bound to the round before randomness is generated. This prevents anyone from generating multiple random values and choosing a favorable one.

Staleness Check

VRF results expire after 300 Solana slots (approximately 2 minutes). If a VRF result is older than this, it is rejected. This prevents "VRF shopping" where someone might try to reuse old random values.

Replay Prevention

Each VRF account can only be used once per round. The program tracks the last VRF account used and rejects duplicates. A monotonically incrementing VRF call counter provides additional replay protection.

Admin Exclusion

Admin wallets are explicitly blocked from participating in games (AdminCannotPlay error). This prevents insider gaming.

3. Card Generation

Bingo cards are generated using a Fisher-Yates shuffle seeded with:

seed = VRF_result XOR player_pubkey XOR slot_index XOR purchase_nonce XOR clock_slot

This ensures every card is unique and unpredictable. The Fisher-Yates shuffle uses rejection sampling to eliminate modulo bias in the shuffle permutation.

4. Win Verification

Win verification is performed entirely on-chain:

1. Every daubed cell is checked against the round's called number bitmask
2. The winning pattern (row, column, diagonal, or blackout) is verified via bitmask comparison
3. Card type restrictions are enforced (some modes require specific patterns)
4. The prize is calculated using on-chain constants with full fee breakdown emitted as an event

5. On-Chain Transparency

Events Emitted

Every significant action emits an on-chain event that you can verify:

• CardPurchased: Player, round, amount, card type
• PrizeCalculatedEvent: Total pool, multipliers, net prize, winner status
• SalaryDistributed: Primary amount, split amount, total distributed
• AdminTaxDistribution: Amount, remaining balance
• TournamentFinished: All prize amounts, total distributed
• PaymentProcessed: Amount, price, product, category

Fee Structure (On-Chain Constants)

6. Contact

Questions about fairness? Reach us at info@fetelabs.com or on X at @BeachBingoSol.